As large-scale energy storage systems are incorporated into such applications as pulsed power and directed-energy, a need for reliable, low maintenance batteries for such applications becomes critical. Consequently there is a need for large, high-rate Li-ion cells that can support a variety of such applications.
In current state-of-the-art battery cell technology, cells capable of high power output are typically limited to small, usually cylindrical formats, as thermal management issues become performance and life-limiting in larger cells. However the use of such small cells in large energy storage systems typically requires large numbers of potentially complex cell interconnects and reduces battery energy densities due to both the high packaging to active material ratio in small cells and the relatively low packing density achievable with cylindrical cells. Conversely large “pouch” cells, which have found increasing use in EV and large energy storage applications are generally limited to relatively low and moderate discharge rates as self-heating in both cell active material and internal tabs creates thermal management issues when these cells are subjected to high charge or discharge rates. In addition, the aluminized Mylar packaging material commonly used in these large cells does not provide the high stack pressure achieved by a “jellyroll” in the metal can of a cylindrical cell. Because of this, an unsupported pouch cell will typically exhibit relatively rapid impedance growth and power and capacity loss as cyclical volume changes in the active materials during operation cause the electrodes to partially delaminate. Consequently to maintain an intimate contact of the cell's active materials over the life of the cell, the Mylar pouch must be reinforced with external mechanical supports. Many applications also require additional reinforcement to protect the cell from mechanical damage.
For conventional cells, both cylindrical and prismatic, a primary path for removing heat from an operating cell is along metal tabs used to carry current out of the cell. This is the case because thermal conductivity parallel to the electrode pairs is many times greater than it is perpendicular to the electrode pairs. Consequently many battery thermal management systems operate by removing heat, either passively or through some active cooling scheme, from the cell interconnects or the battery connections themselves. This strategy may be adequate at moderate discharge rates, but in high power applications with high discharge rates, Joule heating in the cell tabs both internal and external can become significant and the consequent temperature rise in the tabs reduces and, in extreme cases, even reverses the ΔT between the tabs and the cell active material. This may result in excessive heating in the cell active material, which may accelerate material degradation and shorten battery life.